The present invention relates generally to an apparatus and method for radio frequency communication. More particularly, the invention relates to methods and apparatus for isolating radio frequency communication devices and diverting the communication to more acceptable locations.
Since the advent of radio, people have been disturbed by other people's use of such devices. The use of early AM radios were no doubt restricted to low volumes in certain settings. As portability increased, so did the settings in which these devices were viewed with disdain. Boom boxes are not allowed, or at least discouraged, from use in various venues.
In recent years, a new form of communication device has taken center stage—the cellular telephone (“cell phone”). By this time, millions of people in the United States and around the world use cellular phones. One of the most interesting things about a cell phone is that it is actually a radio—an extremely sophisticated radio, but a radio nonetheless.
With the popularity of cell phones increasing, their use in public areas is also increasing. Use in restaurants and elevators, for example, is looked at with contempt by some, but usually not restricted.
Pagers are another wireless device that has seen a surge in popularity recently. Unlike cell phones, their ability to disturb is generally restricted to the audible alert that can accompany a page. However, like cell phones their alerts in the wrong environment can be disconcerting. For example, when they ring in a courtroom, a theater, or a library, the disturbance is substantial.
Likewise, other personal communicators such as personal digital assistants (PDAs), portable email devices and the like are performing more and more wireless communications, most also including an audible alert capability.
Requiring that these devices be disabled in certain environments is currently the best method of stopping these problems. However, in the case of an emergency, this is potentially dangerous where the user is now no longer easily reachable. For example, a doctor would no longer be available to receive a page from a hospital. Further, it requires that the users comply with the request.
There are still other forms of communication devices which have had their use questioned. Various consumer information devices are incorporating a communication standard known as Bluetooth. This, inter alia, allow the devices to communicate with other surrounding Bluetooth devices. Airlines have raise safety concerns over the inability to stop these devices from communicating and their potential for interference with aircraft electronics.
In order to best understand the invention with respect to the common example used herein, some basics of a cellular telephone system must be understood. For that purpose, a prior art cellular system 10 is described in FIG. 1. The cellular system 10 divides a city into small cells. This allows extensive frequency reuse across a city. Because cell phones 12 and base stations 18 use low-power transmitters, the same frequencies can be reused in non-adjacent cells.
Each cell has a base station 18 that consists of a tower 14 and a small building 17 containing the radio equipment 17.
Each cell phone uses two frequencies per call—a duplex channel—so in an analog system, each carrier typically has 395 voice channels and another 42 frequencies for control channels. With digital transmission methods, the number of available channels increases. An analog system will be described, for the purpose of simplicity, though one skilled in the art will realize that the invention works equally with a digital system.
The cellular approach requires a large number of base stations 18 in a city of any size. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO) 20. This office 20 handles all of the phone connections to the normal land-based phone system 22, and controls all of the base stations 18 in the region.
All cell phones 12 have special codes associated with them. These codes are used to identify the phone, the phone's owner and the service provider. The cell phone codes are the following:                Electronic Serial Number (ESN)—a unique 32-bit number programmed into the phone when it is manufactured        Mobile Identification Number (MIN)—a 10-digit number derived from the user's phone's number        System Identification Code (SID)—a unique 5-digit number that is assigned to each carrier by the FCC        
While the ESN is considered a permanent part of the phone 12, both the MIN and SID codes are programmed into the phone 12 when a user purchases a service plan and have the phone activated.
When a user turns on the cell phone 12 and someone tries to call the user, the following occurs:                a. When the user first powers up the phone 12, the phone 12 listens for an SID on the control channel. The control channel is a special frequency that the phone 12 and base station 18 use to talk to one another about things like call set-up and channel changing. If the phone 12 cannot find any control channels to listen to, it knows it is out of range and displays a “no service” message.        b. When the phone 12 receives the SID, the phone 12 compares it to the SID programmed into the phone 12. If the SIDs match, the phone 12 knows that the cell it is communicating with is part of its home system.        c. Along with the SID, the phone 12 also transmits a registration request, and the MTSO 20 keeps track of the user's phone's location in a database. This way, the MTSO 20 knows which cell the user is in when it wants to ring the user's phone 12.        d. The MTSO 20 gets the call, and it tries to find the user. It 20 looks in its database to see which cell the phone 12 is in.        e. The MTSO 20 picks a frequency pair that the user's phone 12 will use in that cell to take the call.        f. The MTSO 20 communicates with the user's phone 12 over the control channel to tell it which frequencies to use, and once the user's phone 12 and the base station 18 switch on those frequencies, the call is connected. The user is talking by two-way radio to a friend.        
As the user travels, the signal is passed from cell to cell.